A 3' LTR- cell DNA junction fragment from a MoMuLV Induced rat thymoma has been cloned in bacteriophage lambda. Tumor cell DNA from 16 tumors has been examined for DNA rearrangements within the cellular sequences represented in this clone. We have shown that in five out of sixteen tumors these cellular sequences have been rearranged due to virus integration. The mechanism by which virus integration operates in tumor induction in this system is other than "promoter insertion."